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On Oct. 19, 2014, Mars is going to get a very close encounter with a comet. On that date—at 18:21 UTC to be precise—C/2013 A1, also known as comet Siding Spring, will pass an incredible 132,000 kilometers (82,000 miles) from Mars. That’s a planetary squeaker, a near miss that, on a cosmic scale, is the thinnest of razor slices.
It’ll definitely miss the Red Planet; we know that for sure. But comets aren’t asteroids; if an asteroid misses, it misses. A comet, though, is a collection of rock, gravel, and dust held together by a variety of ices, and as the comet approaches the Sun that ice sublimates, turns directly into a gas. As that happens other debris dislodges from the comet, forming the fuzzy head and long, sweeping tail. All this debris together can be thousands of kilometers across or more.
So a miss by the relatively small solid nucleus of the comet is good, but the debris cloud may still pose a threat. Right now, Siding Spring isn’t terribly active—it’s spewing out roughly 50 liters of water ice per second, along with dust and probably larger (millimeter sized?) grains of rock. The bulk of this detritus is likely to miss the planet as well, but even if the edge of it skims Mars, it could put some of our robotic assets there at risk.
To minimize the risk, NASA has adjusted the orbits of its Martian orbiters so that they will be on the other side of the planet, protected by its bulk, when the incoming threat from the comet’s dust is predicted to be at its peak. Mars Odyssey orbiter adjusted its orbit on Aug. 5, and Mars Reconnaissance Orbiter made a similar maneuver on July 2. Planners with ESA are looking into what to do with Mars Express to ensure its safety.
What about the rovers and landers? They should be pretty safe; Mars has an atmosphere, though thin, and it should prevent the small bits of cosmic junk from impacting the ground. Larger bits are more rare, and it’s unlikely that, if there are impacts, anything that big will hit anywhere near the rovers. I’d love to see a few smaller impacts close enough to observe with Curiosity, say, but far enough to be safe! That would be fascinating.
I’ll note there are a couple of probes on the way to Mars (MAVEN and MOM). MAVEN arrives just before the comet does, and controllers will adjust its orbit to avoid debris. I poked around, but it’s unclear to me what controllers plan to do with MOM.
My pal Karl Battams has more details on this at the Planetary Society blog, and JPL has a part of its site dedicated to the cometary encounter. This will be something to keep our eyes on for sure.